Alkaline‐Earth‐Catalysed Cross‐Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends,Scope and Mechanism

Alkaline‐earth (Ae=Ca, Sr, Ba) complexes are shown to catalyse the chemoselective cross‐dehydrocoupling (CDC) of amines and hydrosilanes. Key trends were delineated in the benchmark couplings of Ph₃SiH with pyrrolidine or tBuNH₂. Ae{E(SiMe₃)₂}₂ ? (THF)_x (E=N, CH; x=2–3) are more efficient than {N^N}Ae{E(SiMe₃)₂} ? (THF)_n (E=N, CH; n=1–2) complexes (where {N^N}^?={ArN(o‐C₆H₄)C(H)=NAr}^? with Ar=2,6‐iPr₂‐C₆H₃) bearing an iminoanilide ligand, and alkyl precatalysts are better than amido analogues. Turnover frequencies (TOFs) increase in the order Ca<Sr<Ba. Ba{CH(SiMe₃)₂}₂ ? (THF)₃ displays the best performance (TOF up to 3600 h^?1). The substrate scope (>30 products) includes diamines and di(hydrosilane)s. Kinetic analysis of the Ba‐promoted CDC of pyrrolidine and Ph₃SiH shows that 1) the kinetic law is rate=k[Ba]¹[amine]⁰[hydrosilane]¹, 2) electron‐withdrawing p‐substituents on the arylhydrosilane improve the reaction rate and 3) a maximal kinetic isotopic effect (k_SiH/k_SiD=4.7) is seen for Ph₃SiX (X=H, D). DFT calculations identified the prevailing mechanism; instead of an inaccessible σ‐bond‐breaking metathesis pathway, the CDC appears to follow a stepwise reaction path with N?Si bond‐forming nucleophilic attack of the catalytically competent Ba pyrrolide onto the incoming silane, followed by rate limiting hydrogen‐atom transfer to barium. The participation of a Ba silyl species is prevented energetically. The reactivity trend Ca<Sr<Ba results from greater accessibility of the metal centre and decreasing Ae?N_amide bond strength upon descending Group 2.     

Phase equilibria and excess properties for binary systems in reactive distillation processes Part I. Methyl acetate synthesis

Isothermal vapor–liquid equilibrium (VLE) and excess enthalpy (H^E) data were measured for binary systems required for the design of reactive distillation processes for the methyl acetate production. The isothermal P–x data were measured with the help of a computer-operated static apparatus. A commercial isothermal flow calorimeter was used for the determination of the heats of mixing. Temperature-dependent interaction parameters for the UNIQUAC model were fitted simultaneously to the experimental data from this work and other authors.     

Cp*(2)TiMe(2): an improved catalyst for the intermolecular addition of n-alkyl- and benzylamines to alkynes

Cp(2)TiMe(2) has been found to be a competent catalyst for the intermolecular addition of sterically less demanding n-alkyl- and benzylamines to internal alkynes. In the presence of 2.0-6.0 mol % of the catalyst, hydroamination reactions between n-propyl-, n-hexyl-, benzyl-, p-methoxybenzyl- or 2-phenylethylamine and diphenylacetylene, 3-hexyne or 4-octyne go to completion within 24 h or less at 114 degrees C (oil bath temperature). After subsequent reduction of the initially formed imines with zinc-modified sodium cyanoborohydride in MeOH at 25 degrees C, the corresponding secondary amines can be isolated in excellent yields (>78%). Hydroamination/reduction sequences employing the unsymmetrically substituted alkyne 1-phenylpropyne give access to mixtures of regioisomeric secondary amines. The observed regioselectivity is low.     

Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene: a density functional investigation of the influence of electronic and steric factors on the regulation of the selectivity

We present a comprehensive theoretical investigation of the influence of the ligand L on the regulation of the product selectivity for the [Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene. The investigation was based on density functional theory (DFT) and a combined DFT and molecular mechanics (QM/MM) approach for the real [bis(butadiene)Ni(0)L] catalysts with L = PMe(3), I; PPh(3), II; P((i)Pr)(3), III; and P(OPh)(3), IV. The role of electronic and steric effects has been elucidated for all crucial elementary steps of the entire catalytic cycle. Allylic isomerization, allylic enantioface conversion, as well as oxidative coupling are shown to be influenced to a minor extent by electronic and steric effects. In contrast, the ligand's properties have a distinct influence on the preestablished equilibrium between the eta(3),eta(1)(C(1)) and bis-eta(3) forms 2 and 4, respectively, of the [(octadienediyl)Ni(II)L] complex and on the rate-determining reductive elimination following competing routes for generation of either VCH, cis-1,2-DVCB, or cis,cis-COD. Electronic factors are shown to predominantly determine the position of the kinetically mobile 2 right harpoon over left harpoon 4 equilibrium. 4 is the prevailing species for ligands L that are pi-acceptors (L = P(OPh)(3)) or weak sigma-donors (L = PPh(3)), while stronger sigma-donors (L = PMe(3), P((i)Pr)(3)) displace the equilibrium to the left. Steric bulk on the ligand as well as its pi-acceptor ability act to facilitate the reductive elimination, while sigma-donor abilities serve to retard this process. Electronic and steric factors are found to not influence uniformly the reductive elimination routes with either 2 or 4 involved. The regulation of the product selectivity is elucidated on the basis of both thermodynamic and kinetic considerations.     

2,3,5,6-Tetramethylmorpholine: II—1H n.m.r. structural studies of the isomers of 2,3,5,6-tetramethylmorpholine

The structures of the six isomers of 2,3,5,6-tetramethyl-morpholine have been determined by means of proton nuclear magnetic resonance studies at 100 MHz. The spectra have been analysed in terms of spin–spin coupling constants and population distributions of the possible conformers at low temperatures. Also included in this paper are results from n.m.r. studies on the six 4-benzyl-2,3,5,6-tetramethylmorpholine derivatives, which give information about the stereochemistry of the methyl groups α to the amine group.     

Molecular adjustment of the electronic properties of nanoporous electrodes in dye-sensitized solar cells

Molecular modification of dye-sensitized, mesoporous TiO2 electrodes changes their electronic properties. We show that the open-circuit voltage (V(oc)) of dye-sensitized solar cells varies linearly with the dipole moment of coadsorbed phosphonic, benzoic, and dicarboxylic acid derivatives. A similar dependence is observed for the short-circuit current density (I(sc)). Photovoltage spectroscopy measurements show a shift of the signal onset as a function of dipole moment. We explain the dipole dependence of the V(oc) in terms of a TiO2 conduction band shift with respect to the redox potential of the electrolyte, which is partially followed by the energy level of the dye. The I(sc) shift is explained by a dipole-dependent driving force for the electron current and a dipole-dependent recombination current.     

Indications about the shape of the universe from the Wilkinson microwave anisotropy probe data

It is shown that the recent observations of NASA's Explorer mission, "Wilkinson Microwave Anisotropy Probe," hint that our Universe may possess a nontrivial topology. As an example we discuss the Picard space which is stretched out into an infinitely long horn but with finite volume.